Ants On Rafts

Fire ant raft
Photo: PNAS

Fire ants are native to areas that flood and respond to flooding by making rafts. The rafts form as the result of stereotypic behaviors of individuals. Fire ants have larger workers (majors) and smaller workers (minors). Individual ants in water will float on their ventral side and paddle with their legs. They are only capable of paddling in the forward direction. The majors are strong paddlers that do not stop on contact. When two majors collide, they often push each other apart by paddling rather than forming a raft.

Upon contact from another worker, a minor will stop paddling and grab the worker. This attachment starts the raft formation. Minors can attach to either majors or other minors. When the raft reaches about a dozen workers, the ants on the outside will paddle; those in the interior grasp each other. The raft grows as it contacts additional workers. Major workers will climb on the raft, then walk to the edge and begin paddling when their legs touch the water. This causes the raft to grow in a circular pattern with majors on the outside and minors in the spaces between the majors. Queens are found on the top of the raft. The rafts are stable enabling a fire ant colony to stay afloat until the water recedes or the raft contacts the shore.